1. Field of the Invention
The present invention relates to a recording medium capable of reversibly and optically recording data in a very large capacity. Particularly, it relates to a recording medium by use of a polymer liquid crystal compound.
2. Related Background Art
Nowadays, optical recording systems have been put into practical use because of their large capacity and superior capability of random access. A variety of methods are used for the systems, that is, digital audio disks (CD) or laser video disks (LD) have been put into practical use as mediums exclusively used for reproduction. Writing on optical disks (WO) or optical cards (OC) are known as systems by which optical recording can be carried out, including those in which a phase change of a metal thin film is used or those in which a pit form of an organic dye is used.
Researches on rewriting optical disks are also in progress, where attempts are made on practical utilization of those in which magnetooptic effect is used, or those in which a phase change is used. Among them, polymer liquid crystals are also proposed as recording mediums (Japanese Patent Laid-Open No. 59-10930, No. 59-35989 and No. 62-154340). In particular, as a recording system, a system is also proposed in which helical pitch length of a cholesteric polymer liquid crystal is changed or light reflectance is changed in a multiple-value order by the formation of pits in the state of no alignment (Japanese Patent Laid-Open No. 62-107448 and No. 62-12937).
In the above recording mediums, when glass or plastic is used as a substrate, fine tracks must be formed on the substrate so that recording can be stored in a large capacity. Methods of forming tracks on a substrate include a method in which a monomer is coated on a flat substrate, the resulting substrate is made into a given form by the use of a track forming stamper, the monomer is thereafter polymerized, and then the stamper is removed to give a substrate having fine tracks a method in which a resin for forming a substrate is injection-molded into a mold having a stamper forming tracks; and a method in which a substrate is etched to form tracks.
When, however, a recording layer containing a polymer liquid crystal is provided on a substrate having fine tracks to form a recording medium and tracking (i.e., for recording/reproducing/erasing) is carried out thereon, as in the above prior art, there has been the disadvantage that the difference in the refractive index between a material of the substrate or a material that constitutes tracks and the recording layer containing a polymer liquid crystal is so small that no satisfactory tracking can be carried out. As other methods, in order to achieve satisfactory tracking, a groove is formed with a large depth, or a layer to increase reflectance, specifically a semitransparent metal film or a film of a compound with a high refractive index, is provided between a substrate on which tracks have been formed and a recording layer containing a polymer liquid crystal. These methods, however, increase a production cost and lower a contrast and so forth, disadvantageously.